An example of a conventional bridge abutment construction is shown in FIG. 1 generally at 30. Bridge abutment construction 30 includes piles 34 embedded in the ground 38 and supporting an abutment pile cap/wall 42. In a concrete box girder bridge configuration, a concrete deck slab 50 is supported on a concrete girder 46. An open cell construction 46 is supported by the cap 42 with elastomeric bearings 48 between the open cell construction and the cap. Typically, exterior shear keys 54 provide transverse support for the bridge superstructure under seismic loads.
FIG. 2 shows generally at 60 another bridge abutment construction of the prior art, which in addition to exterior shear keys 64 has interior shear keys 68. An end diaphragm 72 transfers the vertical bridge reactions to abutment pile/cap/wall 76, encircles and covers the interior shear keys 68, and overlaps the exterior shear keys 64 with a gap 78 to prevent vertical force transfer. Elastomeric bearings 80 are positioned between the end diaphragm 72 and the abutment pile cap/wall 76. Similar to FIG. 1, piles 84 are embedded in the ground 86 and support the bridge superstructure.
FIG. 3 illustrates the bridge abutment construction of FIG. 1, after having been subjected to a transverse seismic force (EQ). The concrete in the pile cap 42 may fail as shown at 90 and the piles 34 may also fail as shown at 92, depending on the magnitude of the earthquake and their relative capacities. See, e.g., Megally, S. H., Silvia, P. F., and Seible, F., “Seismic Response of Sacrificial Shear Keys in Bridge Abutments,” Structural Systems Research Report SSRP-2001/23, Department of Structural Engineering, University of California, San Diego, La Jolla, Calif., May 2001. (This publication and all other publications and patents mentioned anywhere in this disclosure are hereby incorporated by reference in their entireties.)
It has been recognized that exterior shear keys can be designed as locking mechanisms that limit the magnitude of bridge transverse displacement, yet are intended to yield under a load and limit forces that can be transmitted into the abutment, thereby protecting the abutment piles from severe damage. Thus, the shear keys are expected to act as sacrificial elements to limit the transverse inertial forces in the abutment walls and the supporting piles. These sacrificial shear keys are to be designed with consideration of their over-strength to ensure that the other bridge abutment elements and the piles remain elastic under their highest anticipated capacity associated with their yielding.
FIG. 4 shows in simplified form and generally at 94 an exterior shear key construction of the prior art wherein the exterior shear keys act as sacrificial shear keys. Shear key construction 94 is described in greater detail in Bozorgzadeh, A., Megally, S. H., Ashford, S., and Restrepo, J. I., “Seismic Response of Sacrificial Exterior Shear Keys in Bridge Abutments,” Structural Systems Research Report SSRP-04/14, Department of Structural Engineering, University of California, San Diego, La Jolla, Calif., October 2007 (hereinafter “Bozorgzadeh 2007 Report”). This drawing figure shows a stepped bottom surface 96 and a steel reinforcement 98. The failure mechanism of shear key construction 94 is due to excessive local elongation and local bending of the steel reinforcement associated with very limited concrete block sliding.
The “layer” at the bottom portion of the slanted edge of the shear key in Figure A4-27 of the Bozorgzadeh 2007 Report, but not illustrated in FIG. 4 herein, depicts the constant point of contact assumed to be at a level equal to the height of the bridge bearings. The yielding mechanism of the shear key in the Bozorgzadeh 2007 Report is primarily tension associated with rotation and sliding of the vertical bars in the shear key. The contact area is forced to be along the side face of the girder as the bridge superstructure moves in a translational mode.
Examples of other prior publications that mention shear keys are: U.S. Patent Publications No. 2001/0029711 (Kim et al) and No. 2003/0126695 (Barrett et al); and Bozorgzadh, A., Megally, S., Restrepo, J. A., and Ashford, S. A., “Capacity Evaluation of Exterior Sacrificial Shear Keys of Bridge Abutments,” Journal of Bridge Engineering, September/October 2006, pages 555-565; and Ashford, Scott A., “Abutment Session,” Caltrans & PEER Seismic Research Seminar, Jun. 8, 2009.